Optical communication systems are known that transmit optical signals that carry data as a series of bits. Such optical signals may be coded in accordance with a variety of different formats. Two commonly used formats are referred to return-to-zero (RZ) and non-return-to-zero (NRZ). In accordance with the RZ format, the optical signal returns or transitions to zero or a low power level between each bit or symbol (an “RZ transition”), even if a number of consecutive zeros or one bits occur in the signal. Since the signal returns to zero between each bit, a separate clock signal is, typically, not needed in the RZ signaling scheme.
Non-return-to-zero (NRZ) refers to a signaling scheme in which logic highs are represented by one significant condition and logic lows are represented by another significant condition with no neutral or rest condition, such as zero or lower power level. Since there is no rest state between successive bits, a synchronization signal is typically sent along with the data.
As generally understood, RZ formatted optical signals may have greater tolerance for low optical signal-to-noise ratio (OSNR), and/or high polarization mode dispersion (PMD). In one conventional approach, the optical signals are shaped into RZ formatted pulses with a so-called pulse carver, which may include a Mach-Zehnder modulator (MZM), for example. The MZM is provided in addition to other modulators required to modulate the optical signals with data, thereby adding to system cost and increasing the loss experienced by the transmitted optical signals.
In another approach, so-called electronic RZ or ERZ signals are used to drive the data modulators, thereby eliminating the need for an additional pulse carving modulator.
In order to further increase the data carrying capacity of optical communication systems, however, polarization multiplexing techniques have been employed in which data is modulated onto optical signals having the same wavelength but different polarizations, such as TE and TM polarizations. The optical signals are then combined onto an optical communication path, such as an optical fiber. If so-called advanced modulation formats are employed, such as differential quadrature phase shift keying (DQPSK), both the TE and TM optical signals, or portions thereof, may be supplied to the same photodetector or pair of balanced photodetectors. Accordingly, the TE and TM polarized optical signals may interfere or interact with each in such a way to create errors in the detected bits. In particular, it has been reported certain system impairments are mitigated if the RZ transitions of the TE and TM polarized optical signals are temporally aligned with each other, while other impairments are mitigated if such RZ transitions are temporarily spaced from one another or interleaved. See S. Chandrasekhar et al., “Experimental Investigation of System Impairments in Polarization Multiplexed 107-Gb/s RZ-DQPSK,” Optical Fiber Communication Conference, 2008, the entire contents of which are incorporated herein by reference.
Accordingly, there is a need to control the timing of the RZ transitions between the TE and TM polarized optical signals in a polarization multiplexed optical communication system in order to achieve improved performance.